Hot melt adhesives are applied to a substrate at elevated temperature in a melt form in the substantial absence of a solvent. Upon contact with a substrate the hot melt adhesive can cool, solidify, and form a strong bond. Hot melt adhesives can be both pressure sensitive and non-pressure sensitive depending on formulation. In the past, hot melt adhesives have been made predominantly from hydrophobic or water-resistant components and in that form have satisfied many adhesive requirements. However, the hydrophobic or water-resistant nature of the adhesives have many drawbacks. First, the adhesives tend to be insoluble in water and can be difficult to recycle in aqueous systems. Further, such hydrophobic adhesives tend to be soluble in non-polar solvents. Further, adhesives that are activated by moisture or easily cleaned using aqueous systems are difficult to make from hydrophobic components.
The needs of a number of adhesive applications have not been fully met by typical hot melt adhesives. For example, typical hot melt adhesives have insufficient adhesion to fluorocarbon materials, and polyester materials. Water-soluble, water-sensitive, water-activated adhesives have potential applications and a variety of end uses including label stock, envelope gum, "POST-IT".TM. notes, stamp gum, bindery tapes, sealing tapes, repulpable adhesives, general pressure-sensitive hot melts, non-pressure-sensitive hot melts, carton sealing adhesives, paper-to-paper adhesives and contact lens blocking or mounting adhesives.
The most common water-sensitive or water soluble hot melt adhesives are based on polymers containing vinyl pyrrolidone or other vinyl heterocyclic monomers as taught by Colon et al. in U.S. Pat. Nos. 4,331,576 and 4,325,851. These adhesives are made from vinyl pyrrolidone polymers, vinyl pyrrolidone/vinyl acetate copolymers, and other vinyl pyrrolidone based polymers. Another class of water-sensitive hot melt adhesives include the polyester-based adhesives typically comprising a copolyester in combination with a plasticizer, which are disclosed in Morrison, U.S. Pat. No. 4,052,368 and in Harrington, Jr., et al., U.S. Pat. No. 4,172,824.
The polyvinyl pyrrolidone adhesive can be applied as solvent-free liquids to an appropriate substrates from a reservoir or "glue pot" heated to temperatures of up to about 150.degree.-200.degree. C. Water-sensitive adhesives based on a polyvinyl pyrrolidone or a polyvinyl pyrrolidone/vinyl acetate base often exhibit poor thermal stability. Such adhesive formulations can rapidly degrade (in less than 24 hours) at the elevated pot temperatures which are employed during their application. Further, such adhesives can exhibit inadequate adhesive strength and slow bond set.
Therefore, a substantial need exists for a hot melt adhesive composition that can be substantially heat stable, water-sensitive, can form strong permanent bonds, can be formulated to be moisture-activated or pressure-sensitive, and can be readily solubilized or dispersed in water so that it can be cleaned or repulped in common recycling processes.